Generally, a phosphor is used in light-emitting devices of display devices such as a vacuum fluorescent display (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode-ray tube (CRT) and a light emitting diode (i.e., a LED device).
In these light-emitting devices, it is necessary to supply certain energy to the phosphor in order to excite the phosphor and cause the phosphor to emit light. The phosphor is excited by an excitation source with high energy, such as a vacuum ultraviolet ray, an ultraviolet ray, an electron ray or a blue light and is caused to emit visible light.
In the light-emitting devices using related art phosphors, such as silicate phosphor, phosphate phosphor, aluminate phosphor and sulfide phosphor, however, there has been a problem that brightness of the phosphor is impaired when exposed to the excitation sources described above.
It has been desirable to provide a phosphor of which brightness is not impaired when exposed to the excitation sources described above. An oxynitride phosphor, such as a sialon phosphor, has been proposed as a phosphor of which brightness is hardly impaired.
Patent Document 1 discloses a sialon phosphor including Ca. The sialon phosphor is produced in a process described below. Silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3) and europium oxide (Eu2O3) are first mixed at a predetermined molar ratio. The obtained mixture is sintered through hot pressing. In particular, the mixture is kept in a one atmosphere (i.e., 0.1 MPa) nitrogen atmosphere for one hour at a temperature of 1700° C.
An α-sialon phosphor, in which Eu ions are dissolved, obtained in this process is excited by 450 to 500-nm blue light to emit 550 to 600-nm yellow light. A blue LED device and this phosphor are combined to produce a white LED device.
Patent Document 2 relates to another sialon phosphor and discloses a β-sialon phosphor which has a β-Si3N4 structure. This β type sialon phosphor is excited by near-ultraviolet to blue light to emit 500 to 600-nm green to orange light and is thus suitably used as a phosphor for white LED devices.
Patent Document 3 discloses an oxynitride phosphor of a JEM phase. This oxynitride phosphor is excited by near-ultraviolet to blue light to emit light having a light emission wavelength peak in 460 to 510 nm. The excitation and light emitting wavelength regions of the disclosed phosphor are suitable for a blue phosphor incorporated in a white LED excited by a near-ultraviolet LED.
A white light emitting diode in which a blue light emitting diode device and a blue light-absorbing yellow light-emitting phosphor are combined is well-known as a related art light-emitting device used for a lighting device. This white light emitting diode is practically used for various lighting.
For example, Patent Document 4 discloses a white light emitting diode in which a blue light emitting diode device and a blue light-absorbing yellow light-emitting phosphor are combined. Patent Document 5 discloses a light emitting diode of a similar configuration. Patent Document 6 discloses a light-emitting device in which a wavelength converting casting material is used in a light emitting diode of a similar configuration.
A phosphor used especially frequently in these light emitting diodes is a cerium-activated yttrium aluminum garnet-based phosphor represented by a general formula:(Y, Gd)3(Al, Ga)5O12:Ce3+.
Patent Document 7 discloses a light emitting diode with a phosphor. The light emitting diode includes a semiconductor light-emitting device emitting ultraviolet light or near-ultraviolet light and a phosphor. In this light emitting diode with a phosphor, the semiconductor light-emitting device emits ultraviolet light or near-ultraviolet light by a pulse-form high current. The light emitted from the semiconductor light-emitting device excites the phosphor formed as a film on a surface of the semiconductor light-emitting device. In this configuration, the light emitting diode with a phosphor can emit either of blue light, green light or red light in accordance with a type of the phosphor formed on the surface of the semiconductor light-emitting device.
Patent Document 8 discloses a dot matrix display device which includes a light emitting layer and three types of phosphor layers. The light emitting layer consists of a group III nitride semiconductor. The phosphor layers receive ultraviolet light with a 380-nm peak wavelength emitted from the light emitting layer and each emits trichromatic red, green or blue light.
Patent Document 9 discloses a semiconductor light-emitting device which emits white light using a semiconductor light-emitting device emits 390 to 420-nm light and a phosphor excited by the light emitted from the semiconductor light-emitting device. Since the semiconductor light-emitting device emits light which is not highly visible for human being, a viewer hardly feels variation in a color tone due to any change in light emitting intensity and a light emission wavelength of the semiconductor light-emitting device. The 390 to 420-nm light hardly damages device components, such as resin in which a phosphor is dispersed. Although ultraviolet light generally has various harmful influences on human bodies, light at a wavelength of not less than 390 nm has no harmful influence caused by escaping excitation light. Examples of the phosphor excited by 390 to 420-nm light to emit light include various oxide phosphors and sulfide phosphors.
Such a lighting device can be fabricated by publicly known methods as disclosed in, for example, Patent Documents 10 and 11.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-363554
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-255895
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2006-232868
[Patent Document 4] Japanese Patent No. 2900928
[Patent Document 5] Japanese Patent No. 2927279
[Patent Document 6] Japanese Patent No. 3364229
[Patent Document 7] Japanese Unexamined Patent Application, First Publication No. 1998-12925
[Patent Document 8] Japanese Unexamined Patent Application, First Publication No. 1997-153644
[Patent Document 9] Japanese Unexamined Patent Application, First Publication No. 2002-171000    [Patent Document 10] Japanese Unexamined Patent Application, First Publication No. H5-152609
[Patent Document 11] Japanese Unexamined Patent Application, First Publication No. H7-99345
The phosphors disclosed in Patent Documents 1 to 11 have an excitation spectrum and a light emission spectrum suitable for a white LED excited by a near-ultraviolet to blue light device. However, the advent of a phosphor having still higher brightness has been desired due to a recently increasing demand for a still brighter white LED.
Regarding light-emitting devices, such as lighting, there is a problem that a white light emitting diode formed by a blue light emitting diode device and an yttrium aluminum garnet-based phosphor is characterized by emitting bluish white light due to shortage of a red component and thus the color rendering becomes ununiform. In such a white light emitting diode, especially an oxide phosphor has a low covalent bond property. Accordingly, there has been a problem that light emission brightness is impaired due to an increase in the heat generation rate of a high-brightness semiconductor light-emitting device.
A sulfide-based phosphor is known to have high light emission brightness, but is chemically unstable. It is therefore difficult to provide a long-life characteristic inherent to a white LED.